Light moves in not-so-mysterious ways

Marie Freebody, marie.freebody@photonics.com

How can we search the Internet more quickly? How will future quantum computer networks operate? What is happening during the fast energy transfer in photosynthesis? These and many other questions related to the dynamics of complex systems are now being explored, thanks to an all-optical implementation of a quantum walk devised by researchers in Germany in collaboration with theorists Vaek Potock, Aurel Gábris, Erika Andersson and Igor Jex, all from Czech Technical University in Prague.

A quantum walk is the path a single photon of light takes as it travels through a network. If you or I were to take a walk and came to a split in the path, we could walk down only one path at a time. Photons, on the other hand, have the intriguing ability to travel down both paths at the same time, as demonstrated in a simple experiment carried out at Max Planck Institute for the Science of Light (MPL).

In this illustration of the MPL setup, a pulse of light (red arrow on the right) is coupled into the network loop. First, the polarization of the photon is controlled by an optical element, which corresponds to apply a quantum coin. The photon then travels through an optical fiber (yellow) that presents a shorter and a longer path. Two mirrors are used to send the photon back to the in/out coupler, after which the photon is probabilistically sent back to the coin, thus repeating the loop (step of the walk). If the photon is coupled out of the setup, its arrival time at the detector is registered. Images courtesy of Max Planck Institute for the Science of Light.

Dr. Christine Silberhorn, group leader at the MPL, together with Dr. Katiuscia N. Cassemiro, Dr. Peter Mosley and Andreas Schreiber, have realized five steps of a quantum walk carried out by a single photon. The experiments could provide new insight into statistical processes such as photosynthesis and help to accelerate search algorithms.

“To date, only a few experimental realizations of quantum walks have ever been reported. A major problem faced by the different optical implementations is to achieve interferometric stability and scalability,” Schreiber said. “In contrast to other implementations, we have controlled the motion of the walker, observing behaviors in which the photon could be found in any of the final positions or even forcing it to follow a single path.”

Doctoral student Andreas Schreiber is working hard to get the experimental results in the MPL laboratory.

This control is fundamental for any quantum computational application, and what’s more, the MPL group has developed a setup that is highly stable and scalable, thanks to its conceptual idea based on an optical feedback loop.

“It has been shown that a quantum walker can reach any site of the network much faster than a walker obeying classical physics,” Cassemiro said. “In the same way that random walk is important to classical computation, its quantum version is now the focus of studies concerning its applicability to the design of quantum computers.”

In the MPL experiment, which was reported in the February 2010 issue of Physical Review Letters, a photon, traveling at the speed of light, passes through a sequence of mirrors and other elements that form a large loop, which includes a split at one point. One path takes a few nanoseconds longer to travel around compared with the other, shorter path.

This time difference is enough to be reliably distinguished by a detector after the photon traverses several repeated loops. By repeating the experiment millions of times, the MPL team observed that the particle follows quantum laws while inside the loop.

“The light particle doesn’t really choose which path to take when faced with the split in the circuit,” Cassemiro said. “Instead, it takes both paths simultaneously, creating a so-called quantum superposition. The various paths that the photon can take with the same escape time leads to quantum interference, which constitutes the principal difference between the quantum and classical walk.”

Because after each step of the walk the walker is sent back to the same loop, no further components are needed to increase the number of steps, thus ensuring the scalability. Moreover, the fact that the same optical paths are used in all steps guarantees the stability or coherence of the setup.

The ultimate goal of the MPL team is the realization of quantum algorithms based on quantum walks. In the meantime, it is upgrading its experiments to allow the quantum walk to perform a considerably larger number of steps and is working toward controlling the precise behavior of the particles – essential for performing simple algorithmic tasks.